1. Field of the Invention
The present invention is related to a method of performing an eye examination utilizing optical coherence tomography (OCT).
2. Discussion of Related Art
Retinal imaging by conventional optical image methodology, such as fundus camera imaging and indirect ophthalmoscopic imaging, has been routinely used clinically to evaluate retinal structure change. Routine retinal imaging provides valuable information for a clinician to diagnosis a number of eye diseases, including glaucoma. When there is a need to evaluate the optic nerve head tissue structure changes for glaucoma patients, stereoscopic retinal images are required to detect volumetric changes in the three dimensional nerve head structure. However, to date an experienced clinician can only provide a qualitative interpretation of eye structural changes from the retinal photograph.
Several imaging methods have been explored to quantitatively measure the three-dimensional structure of the nerve head. The Glaucoma Scope made by Ophthalmic Imaging Systems, Sacramento, Calif., used a technique of computed raster stereography. The Glaucoma Scope projected a series of equidistant, parallel, straight line beams of light onto the nerve head at oblique angles. By measuring the amount of deflection of the lines of light, nerve head topography can be determined. From the topographic view of the nerve head, many clinically significant volumetric parameters can be derived, such as disk area, cup area, disk rim area, and retinal nerve fiber layer (RNFL) thickness on the disk margin.
The Heidelberg Retinal Tomography (HRT), produced by Heidelberg Engineering, Germany, is based on a Laser Scanning Ophthalmoscope, SLO. By moving the focus plane of the scanning beam in the SLO, the topography of the nerve head can be measured. However, tissues like the choroid layer, which is underneath the superficial retinal surface layer, can not be seen with the SLO methods. As a result, the topography of the optical nerve layer is indirectly measured utilizing an artificial reference plane. Even with these advanced techniques, the ability to sufficiently map the optic nerve layer is limited. Further, the disk margin, which is also inside the retinal nerve fiber layer, is difficult to be accurately outlined by the SLO image. The accuracy of determining nerve head changes is limited.
A glaucoma exam, GDx, produced by Laser Diagnosis Technology, San Diego, Calif. is another method for mapping the RNFL. The GDx technique is based on polarimetry. The RNFL tissue is birefrigent and will cause polarization rotation as the probing beam of light passes through the RNFL. The thickness of the RNFL is indirectly measured by measuring the magnitude of the polarization rotation as the light beam is scanned across the retina. The RNFL thickness map is obtained by scanning the laser beam on the nerve head region. There are also disadvantages with GDX diagnosis. The cornea tissue is also birefrigent, which will add to the polarization rotation. The magnitude of polarization rotation by the cornea depends on the cornea thickness and light beam incident angle. The RNFL thickness accuracy significantly depends on the individual subjects to be measured.
Optical Coherence Tomography (OCT) is a new image modality that has been used for non-invasive human eye retinal imaging. A cross sectional retinal image taken while the beam is scanned across the retina allows the clinician to quantitatively evaluate the retinal nerve layer and retinal thickness. By composing radial line scan patterns, a 3-D nerve head geometry can be derived. An OCT system produced by Carl Zeiss Meditec, Dublin, Calif., for example, scans six radial lines passing across the nerve head. Volumetric parameters like disk area, cup area, and disk rim area are derived from these radial line images. Conventionally, the RNFL thickness is measured in a circular scan at a diameter of 3.45 mm centered on the center of the disk. OCT is advantageous over previous methods because OCT provides a direct measurement of the tissue thickness and does not significantly depend on other ocular tissue conditions. However, the sampling density is low compare to the other imaging methods and there are artifacts of the measurements resulting from slow scan speeds. Also, the RNFL thickness by a circular scan around the disk is often not reliable due to the off centering of the scan caused by inaccurate visual alignment and eye motion. The complete mapping of the retina nerve head volumetric parameters and RNFL around the nerve head region is usually unobtainable due to eye motion during the scan.
A complete mapping of the nerve head by OCT imaging has been possible only if the eye is fixed without any motion and there is no obscuration of the OCT scan beam so that important nerve head tissue are all visible in the OCT image. However, neither of these assumptions are feasible in a human subject.
Several attempts have been made to track the scan beam with the retina in order to eliminate the effects of eye motion. Dan Ferguson (Physical Science Inc, Andover, Mass.) utilized active feedback to track the scan beam on the retina based on a reflectometry principle. This method provides real-time tracking capability and has potential to scan completely over the nerve head. However, the extra confocal scanning laser hardware that needs to be added to the OCT scanner to perform this tracking method is complicated and expensive. Further, during a blink of the patient's eye, the tracking signal is lost and may not be recoverable from the previous scan sequence.
Another method of compensating for eye motion has been proposed by Dara Koozekanani (The Ohio State University, Columbus, Ohio). This method uses a combination of the reflected signal of the scan beam and a video image to register the retinal position. However, it is unclear as to use of this method for mapping the clinically significant nerve head parameters. Using raster line OCT scans to acquire three dimensional data sets for mapping of the retinal layer thickness has been described by Mujat et al in Optical Express. However, no description of how to map the nerve head boundary contour, which is essential as a reference for deriving all nerve head morphologic parameters, has been provided.
There is a need for direct measurement of all nerve head volumetric parameters, with complete mapping of the RNFL around the nerve head. Further, there is a need for acquiring and displaying all clinically significant information corresponding to the nerve head morphology that are highly desired by clinicians for diagnosing diseases such as glaucoma.